Pozzolan cement is a type of hydraulic cement, meaning it reacts with calcium hydroxide and water to form a water resistant cementitious compound. The use of pozzolanic cements dates back to 500-400 BC, when the ancient Greeks used volcanic ashes.
The benefits of pozzolan cements and concretes are numerous. First, pozzolan materials are generally cheaper than their alternative, Portland cement. Second, the production of pozzolan cements is generally more environmentally friendly than Portland cements. For example, the production of Portland cement requires large amounts of energy, and as a result, enormous amounts of carbon dioxide are produced, along with numerous other pollutants. Third, the addition of pozzolan tends to increase durability of the end product. For example, Pozzolan concretes have been shown to outperform Portland concretes with regard to sulfate attacks and alkaline silicon reactivity attacks. Finally, many of the artificial pozzolans are industrial byproducts, such as blast furnace slag, the usage of which creates value and environmental savings where otherwise none would be.
Despite these advantages, many of the industrial byproduct pozzolans, such as blast furnace slag, are too costly or not always available. Other cheaper and more readily available pozzolans, such as fly ash, are not immediately suitable for use, but must be processed in order to be suitable for use as high quality cement. For instance, it has been shown that milling Class F fly ash to under 45 microns in diameter, results in the production of slag grade 100 or above concrete, as per ASTM C989.
With respect to the oil and gas industry, part of the process of preparing a well for further drilling, production or abandonment is cementing the well. Cementing protects and seals the wellbore. Part of the completion process of a prospective production well, cementing is used to seal the annulus after a casing string has been run in a wellbore. Additionally, cementing is used to seal a lost circulation zone, or an area where there is a reduction or absence of flow within the well. Also, cementing is used to plug a well prior to abandoning it.
Cementing is performed when a cement slurry is deployed into the well via pumps, displacing the drilling fluids still located within the well, and replacing them with cement. The cement slurry flows to the bottom of the wellbore through the casing, which will eventually be the pipe through which the hydrocarbons flow to the surface. From there it fills in the space between the casing and the actual wellbore, and hardens. This creates a seal so that outside materials cannot enter the well flow, as well as permanently positions the casing in place.
Determining the required physical properties of the cement is essential before commencing cementing operations. Special mixers, including hydraulic jet mixers, re-circulating mixers or hatch mixers, are typically used to combine dry cement with water to create the wet cement, also known as slurry. Cement used in the well cementing processes can be one of the 5 different API types or even construction grade cement can be utilized.
Additives to the cement can include accelerators, which shorten the setting time required for the cement, as well as retarders, which do the opposite and make the cement setting time longer. In order to decrease or increase the density of the cement, lightweight and heavyweight additives are added. Nitrogen can be utilized as a means to reduce the density of the cement. Extenders, such as fly ash and sodium silicates, can be used to replace portions of the cement in an effort to reduce the cost of cementing.
The final size of the cement particles has a direct relationship with how much water is required to make a slurry without producing an excess of water at the top of the cement or in pockets as the cement hardens. In other words, the rate at which a cement particle hydrates when exposed to water greatly depends on its size. A small particle reacts much more quickly than a large particle and a very large particle, larger than about 50 μm, probably will never become fully hydrated, even if exposed to enough water. The particle size diameter is therefore critical in controlling the rate at which cements gain strength. The surface area increases inversely as the square of the mean particle diameter, therefore reducing the surface area by a factor of for example, live increases the area by 25, and because the new surface area is chemically fresh, it is more reactive.
Pozzolans consist generally of aspherical particles and spherical particles in the form of aluminio ferro silicate glass beads. Traditional milling techniques simply crush pozzolans, which fails to polish or grind the material. This results in non-active pozzolan particles as compared to rotary milled pozzolan. Using a combination of to rotatory mill with variably sized and shaped media, not only can fly ash be reduced to below 25 microns, but its surface area can be increased from the typical 0.695 m2/g to 1.263 m2/g, thus increasing the reactivity and stability of the resulting fly ash. Furthermore, the treatment described above both reduces the size of the non-spherical particles while at the same time roughing up the spherical particles, thereby increasing the surface area without reducing the flow ability of the pozzolan and results in a concomitant rise in reactivity.
One skilled in the art will recognize that despite increased reactivity and stability, fly ash with a particulate size of 25 microns or below is unsuitable in and of itself for use in oil and gas wells. It would be greatly beneficial to reduce the overall size of a composite cement to reduce the particle size of the entire blend.
Currently such fly ash is combined with other materials to form a composite cement. Often such added materials are of larger particle sizes than 25 microns, thus reducing the reactivity and slurry properties of the blended concrete. Even if the added particles are separately milled to a mean particle size of less than 25 microns, it is still possible to further improve the reactivity, stability, and slurry properties. Using the method described below, one can achieve mean particles sizes of 7 microns or below with surface areas of 153 m2/g.